It is known in the art of refrigeration systems to employ in conjunction with suction accumulators, receivers, and related components fusible plugs in order to relieve gas pressure when the ambient temperature reaches an extremely high level, such as during a fire. In U.S. Pat. No. 3,139,103, which issued June 30, 1964 in the name of one of the present applicants, the prior art relating to this subject is described, and reference is now made thereto for background information.
As is set forth in this patent, and prior to the invention disclosed therein, fusible plugs of the past were generally secured to a refrigeration system component by assembly with a female pipe fitting permanently secured to the component. The fitting had an axial passage therethrough, one end of which was internally threaded for mating engagement with an externally threaded fusible plug. Disadvantageously, due to thread tolerances between the fittings and fusible plugs, deformation of the threads was necessary in order to provide a seal therebetween. This deformation frequently created stresses within the fusible plug, sufficient to cause leaks around the fusible material therein.
The expedient of placing sealant on the threads of the fittings and/or fusible plugs was found to be ineffective. The apparent reason therefore was that the sealing compounds employed often deteriorated on exposure to refrigerant, thereby producing delayed leaks of refrigerant around the fusible plugs.
Leakage and other problems associated with the fusible plug structure just described were effectively eliminated by provision of the fusible plug disclosed in U.S. Pat. No. 3,139,103. As shown therein, the plug has a male flare to connect to a female flare on the receiver or accumulator. A soft copper gasket, which is generally oiled, is used, and pipe compound is eliminated. The result is a permanent leak-tight connection at the threads.
Additionally, in the patented structure, the alloy constituting the fusible material is located so that it is not disturbed or under stress when the plug is tightened. This is because the section of the plug containing the alloy is not the threaded section to which tightening forces are applied. By use of an eutectic alloy and the absence of significant distortion thereof, the relief temperature is positive.
While the fusible plug just described has functioned effectively in actual practice, it has been noted that certain marked improvements can be made therein. First, in some cases, depending upon the positioning of the female fitting attached to the refrigeration system component, it is inconvenient or possibly difficult to properly insert the copper gasket. This gives rise to the possibility of leakage. Second, upon occasion during installation the gasket or seal may rotate relative to the female sealing surface upon the fitting, rather than only upon the male sealing surface on the plug itself. This may cause galling of the copper gasket, since generally the male sealing surface on the plug is more smooth or dimensionally stable, as compared with the female sealing surface on the fitting.